Comment 3 for bug 1315369

One more check to figure out where the problem might be:
 - Ctrl + Alt + F1
 - Login there and run:
     COMPIZ_CONFIG_PROFILE=ubuntu DISPLAY=:0 compiz --replace

At this point, go back with Ctrl+Alt+F7, and there unity should reload. If it resumes your session properly then it seems an unity's fault.

You can also just run "unity" from tty1, but that will relock your session. If compiz doesn't seem to reload try with a "killall -9 compiz" before.

PS: if you want to lower the idle delay needed to blank and lock the screen for testing purposes you can use
  gsettings set org.gnome.desktop.session idle-delay 1 # means 1 second

Use this to reset the default values instead:
  gsettings reset org.gnome.desktop.session idle-delay